Surface-treated steel sheet, metal container, and method for producing surface-treated steel sheet

ABSTRACT

There is provided a surface-treated steel sheet (1) comprising: a tin-plated steel sheet (10) obtained by tin-plating a steel sheet (11); a phosphate compound layer (20) containing tin phosphate formed on the tin-plated steel sheet (10); and an aluminum-oxygen compound layer (30) on the phosphate compound layer (20), a main constituent of the aluminum-oxygen compound layer (30) being an aluminum-oxygen compound; wherein, when the 3 d5/2 spectrum of tin in the aluminum-oxygen compound layer (30) is determined using an X-ray photoelectron spectroscopy, the ratio of the integration value of the profile derived from tin oxide to the integration value of the profile derived from tin phosphate (tin oxide/tin phosphate) is 6.9 or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending application Ser. No.15/546,624, filed on Jul. 26, 2017, for which priority is claimed under35 U.S.C. § 120; and this application claims priority of Application No.2015-012634 filed in Japan on Jan. 26, 2015 under 35 U.S.C. § 119, theentire contents of all of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a surface-treated steel sheet, a metalcontainer, and a method for producing a surface-treated steel sheet.

BACKGROUND ART

Methods for chromate-treating the surface of a base material used in thefield of metal containers, domestic appliances, building materials,vehicles, aircrafts, and the like have been known. Further, non-chromicsurface treatment, replacing such chromate treatment, has beendeveloped. Patent Document 1 discloses, for example, a non-chromicsurface treatment technique of forming a metal-oxygen compound filmcontaining aluminum on a base material surface by means of cathodeelectrolytic treatment using an electrolytic treatment liquid containingaluminum ions.

PRIOR ART DOCUMENT Patent Document

-   [Patent Document 1] JP 2006-348360A

SUMMARY OF INVENTION Problems to be Solved by Invention

Unfortunately, the surface-treated steel sheet obtained by theconventional art described in the above Patent Document 1 may exhibitpoorer corrosion resistance than that of a surface-treated steel sheetof which surface is chromate-treated when used in cans for food andbeverages and the like.

It is an object of the present invention to provide a surface-treatedsteel sheet having excellent corrosion resistance.

Means for Solving Problems

The present inventors have found that the above object can be achievedby a surface-treated steel sheet obtained by forming a phosphatecompound layer containing tin phosphate on a tin-plated steel sheetfollowed by forming an aluminum-oxygen compound layer mainly formed ofan aluminum-oxygen compound on this phosphate compound layer wherein thecontent ratio between tin oxide and tin phosphate contained in thealuminum-oxygen compound layer is adjusted. The inventors have thusaccomplished the present invention.

Specifically, according to an aspect of the present invention, there isprovided a surface-treated steel sheet including: a tin-plated steelsheet obtained by tin-plating a steel sheet; a phosphate compound layercontaining tin phosphate formed on the tin-plated steel sheet; and analuminum-oxygen compound layer formed on the phosphate compound layer, amain constituent being an aluminum-oxygen compound, wherein, when the 3d5/2 spectrum of tin in the aluminum-oxygen compound layer is determinedusing an X-ray photoelectron spectroscopy, the ratio of the integrationvalue of the profile derived from tin oxide to the integration value ofthe profile derived from tin phosphate (tin oxide/tin phosphate) is 6.9or more.

In the surface-treated steel sheet of the present invention, thealuminum-oxygen compound layer preferably contains aluminum phosphate.

In the surface-treated steel sheet of the present invention, the totalamount of phosphorus contained in each of the layers formed on thetin-plated steel sheet is preferably 0.5 to 20 mg/m².

In the surface-treated steel sheet of the present invention, the contentof aluminum in the aluminum-oxygen compound layer is preferably 5 to 15mg/m².

In the surface-treated steel sheet of the present invention, thealuminum-oxygen compound layer preferably contains substantially nofluorine.

In the surface-treated steel sheet of the present invention, thetin-plated steel sheet preferably consists of the steel sheet, a tinalloy layer formed on the steel sheet, and a tin-plating layer formed onthe tin alloy layer, and a total amount of tin in the tin alloy layerand the tin-plating layer is preferably 1.0 g/m² or more.

According to another aspect of the present invention, a metal containerformed of the surface-treated steel sheet is provided.

According to still another aspect of the present invention, there isprovided a metal container including the surface-treated steel sheet anda coating layer formed on the aluminum-oxygen compound layer of thesurface-treated steel sheet, a main constituent of the coating layerbeing an organic material.

According to still another embodiment of the present invention, there isprovided a method for producing a surface-treated steel sheet including:a phosphate compound layer-formation step of forming a phosphatecompound layer on a tin-plated steel sheet by means of cathodeelectrolytic treatment, wherein the tin-plated steel sheet is obtainedby tin-plating a steel sheet; and an aluminum-oxygen compoundlayer-formation step of forming an aluminum-oxygen compound layer on thephosphate compound layer by means of electrolytic treatment using anelectrolytic treatment liquid containing aluminum.

In the phosphate compound layer-formation step of the production methodof the present invention, the phosphate compound layer is preferablyformed on the tin-plated steel sheet by means of cathode electrolytictreatment after anode electrolytic treatment is conducted.

In the production method of the present invention, a treatment liquidhaving a phosphate content of 0.55 g/L or less in terms of phosphorus ispreferably used as the electrolytic treatment liquid in thealuminum-oxygen compound layer-formation step.

Effect of Invention

According to the present invention, a phosphate compound layer and analuminum-oxygen compound layer is formed on a tin-plated steel sheetwhile setting the ratio the ratio of the above tin oxide/tin phosphateto 6.9 or more in the aluminum-oxygen compound layer. Then, there can beprovided a surface-treated steel sheet having excellent corrosionresistance due to action of tin oxide and phosphate such as tinphosphate contained in the aluminum-oxygen compound layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating a configuration of asurface-treated steel sheet according to an embodiment of the presentinvention.

FIG. 2 is a schematic view illustrating a process of forming a phosphatecompound layer and an aluminum-oxygen compound layer on a tin-platedsteel sheet.

FIG. 3 is a graph showing an exemplary result of measurement of analuminum-oxygen compound layer by an X-ray photoelectron spectroscopy(XPS).

FIGS. 4(A) and 4(B) each are views illustrating an example of a metalcontainer formed using the surface-treated steel sheet according to anembodiment of the present invention.

FIGS. 5(A) and 5(B) are graphs showing the result of analysis of analuminum-oxygen compound layer by an X-ray photoelectron spectroscopy(XPS) in Example 5 and Example 6 according to the present invention,respectively.

FIG. 6 includes a cross-sectional view obtained by a transmissionelectron microscope (TEM) illustrating a state where a phosphatecompound layer and an aluminum-oxygen compound layer are formed on atin-plated steel sheet in Example according to the present invention,and the result of quantitative analysis thereof at each point.

MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will hereinafter be described withreference to the figures.

FIG. 1 is a cross-sectional view illustrating a configuration of asurface-treated steel sheet 1 according to an embodiment of the presentinvention. The surface-treated steel sheet 1 of the present embodimentis obtained by the following method: first, a tin-plated steel sheet 10obtained by forming a tin-plating layer 12 on a steel sheet 11 issubjected to electrolytic treatment in an electrolytic treatment liquidcontaining phosphate ions to form a phosphate compound layer 20 on thetin-plated steel sheet 10 while dissolving a portion of the tin-platinglayer 12, and subsequently the tin-plated steel sheet 10 thus subjectedto electrolytic treatment is further subjected to electrolytic treatmentin an electrolytic treatment liquid containing Al ions to form analuminum-oxygen compound layer 30 on the phosphate compound layer 20while dissolving a portion of the phosphate compound layer 20.

The surface-treated steel sheet 1 of the present embodiment may be usedas, but not particularly limited to, a member such as a can container orcan lid, for example. When the surface-treated steel sheet 1 is used asa member such as a can container or can lid, the surface-treated steelsheet 1 may be used as is (used in applications requiring no coating, inwhich no coating layer is formed on the surface) to be shaped into anon-coated can container or can lid. As shown in FIG. 1, the steel sheet1 may also be shaped into a can container or can lid, after a coatinglayer 40 foamed of an organic material is formed on the aluminum-oxygencompound layer 30 of the surface-treated steel sheet 1.

<Tin-Plated Steel Sheet 10>

The tin-plated steel sheet 10, which is to be the base material of thesurface-treated steel sheet 1 of the present invention, can be obtainedby tin-plating the steel sheet 11 to form a tin-plating layer 12 on thesteel sheet 11.

The steel sheet 11 to be tin-plated may be any steel sheet havingexcellent drawing formability, drawing and ironing formability, andformability in drawing and thin redrawing (DTR). For example, there canbe used, but not particularly limited to, a hot-rolled steel sheet suchas one based on an aluminum-killed steel continuously cast material anda cold-rolled steel sheet obtained by cold-rolling such a hot-rolledsteel sheet. As for the steel sheet to be tin-plated, a steel sheet canbe used whose corrosion resistance is improved by forming anickel-plating layer on the steel sheet, heating the resulting steelsheet for thermal diffusion, and forming a nickel-iron alloy layerbetween the steel sheet and the nickel-plating layer. When thenickel-plating layer is formed into a granular form, the adhesion of thecoating layer can be increased by an anchoring effect.

The method for tin-plating the steel sheet 11 is not particularlylimited, and examples thereof include methods using a known plating bathsuch as a ferrostan bath, a halogen bath, and a sulfuric acid bath. Themethod for nickel-plating is also not particularly limited, and a knownWatt bath including nickel sulfate and nickel chloride can be used. Whenthe nickel-plating layer is formed into a granular form, a bathcomposition including nickel sulfate and ammonium sulfate is preferablyused. Furthermore, in the present embodiment, as for the tin-platedsteel sheet 10 obtained by being tin-plated, a tin-iron alloy layer maybe formed between the steel sheet 11 and the tin-plating layer 12 byconducting treatment of heating the tin-plated steel sheet to atemperature equal to or higher than the melting temperature of tinfollowed by rapid cooling (reflow treatment). In the present embodiment,the tin-plated steel sheet 10 obtained by being subjected to this reflowtreatment will be one in which the tin-iron alloy layer and thetin-plating layer 12 are formed on the steel sheet 11 in this order,resulting in improved corrosion resistance. When a nickel-plating layeris present as the base, tin-nickel and tin-nickel-iron alloys may beformed between the steel sheet 11 and the tin-plating layer 12 by thisreflow treatment.

In the present embodiment, the surface of the tin-plated steel sheet 10obtained as described above is generally oxidized by oxygen to form anoxide film layer formed of SnO_(x) (x=1 to 3) thereon. An unduly largeamount of the oxide film layer formed of SnO_(x) tends to reduce theadhesion of the phosphate compound layer 20 formed on the tin-platedsteel sheet 10. In contrast, an unduly small amount of the oxide filmlayer tends to easily cause sulfide blackening of the tin-plated steelsheet 10. Thus, the amount of the oxide film layer is desirably adjustedappropriately. Accordingly, in the present embodiment, treatment toadjust the amount of the oxide film layer by removing a part or all ofthe oxide film layer on the surface may be conducted on the tin-platedsteel sheet 10. For example, treatment to remove the oxide film layer onthe surface of the tin-plated steel sheet 10 may be conducted on thetin-plated steel sheet 10 by conducting at least one of cathodeelectrolytic treatment or anode electrolytic treatment under conditionsof a current density of 0.5 to 20 A/dm² and an energizing time of 0.1 to1.0 seconds using a carbonate alkaline solution such as sodium carbonateand sodium hydrogen carbonate. The oxide film layer may be removed alsousing an acidic aqueous solution such as hydrochloric acid. In thiscase, the immersion time of the tin-plated steel sheet 10 in an acidicaqueous solution is preferably 2 seconds or less. An immersion time ofthe tin-plated steel sheet 10 in an acidic aqueous solution within theabove range allows efficient removal of the oxide film layer formed ofSnO_(x) while reducing dissolution of the metal tin portion of thetin-plating layer 12.

The thickness of the tin-plating layer 12 formed on the steel sheet 11is not particularly limited. The thickness may be selected appropriatelydepending on the intended usage of the surface-treated steel sheet 1 tobe produced, and is preferably 1.0 g/m² or more, more preferably 1.0 to15 g/m² in terms of tin. When a nickel-plating layer is also provided,the thickness of the nickel-plating layer is not particularly limited.The thickness of the nickel-plating layer is preferably 0.01 to 15 g/m²in terms of nickel. When the nickel plating is in the granular form, theaverage particle size of the granular nickel is preferably 0.01 to 0.7μm.

The total thickness of the tin-plated steel sheet 10 is not particularlylimited. The thickness may be selected appropriately depending on theintended usage of the surface-treated steel sheet 1 to be produced, andis preferably 0.07 to 0.4 mm.

<Phosphate Compound Layer 20>

The phosphate compound layer 20, which is a layer containing tinphosphate, is formed by immersing the tin-plated steel sheet 10 in anelectrolytic treatment liquid containing phosphate ions and conductingcathode electrolytic treatment using the tin-plated steel sheet 10 asthe cathode.

In the present embodiment, when the tin-plated steel sheet 10 immersedin an electrolytic treatment liquid containing phosphate ions isenergized as the cathode side, tin dissolves from the tin-plated steelsheet 10 to generate divalent tin ions (Sn²⁺) as shown in the leftfigure in FIG. 2.

FIG. 2 is a schematic view illustrating a process of forming a phosphatecompound layer 20 and an aluminum-oxygen compound layer 30 being formedon a tin-plated steel sheet 10. The left figure in FIG. 2 illustrates astate in which the tin-plating layer 12 of the tin-plated steel sheet 10is subjected to cathode electrolytic treatment using an electrolytictreatment liquid containing phosphate ions. The middle figure in FIG. 2illustrates a state in which Sn₃(PO₄)₂ formed as the phosphate compoundlayer 20 is subjected to cathode electrolytic treatment for forming thealuminum-oxygen compound layer 30. The right figure in FIG. 2illustrates a state in which Sn₃(PO₄)₂ and AlPO₄ as the phosphatecompound layer 20 and AlPO₄, Al₂O₃.nH₂O, and Al(OH)₃ as thealuminum-oxygen compound layer 30 are formed on the tin-plated steelsheet 10. In the present embodiment, AlPO₄ is contained in both thephosphate compound layer 20 and the aluminum-oxygen compound layer 30.

In the present embodiment, as shown in FIG. 2, the tin ions Sn²⁺generated from the tin-plated steel sheet 10 react with phosphate ionsPO₄ ³⁻ in the electrolytic treatment liquid to be deposited on thetin-plated steel sheet 10 as tin phosphate such as Sn₃(PO₄)₂. The tinions Sn²⁺ generated from the tin-plated steel sheet 10 are deposited onthe tin-plated steel sheet 10 also as tin oxide (SnO_(x)).

Phosphates in the aqueous solution are known to have changes in theionization equilibrium among dihydrogen phosphate ions (H₂PO₄ ⁻),hydrogen phosphate ions (H₂PO₄ ²⁻), and phosphate ions (PO₄ ³⁻)depending on the pH of the aqueous solution. As the pH of the aqueoussolution becomes lower, the ionization equilibrium moves towards theside where the abundance ratio of dihydrogen phosphate ions H₂PO₄ ⁻ishigh. In the present embodiment, when the tin-plated steel sheet 10 issubjected to cathode electrolytic treatment with an electrolytictreatment liquid containing phosphate ions, tin oxide formed on thesurface of the tin-plating layer 12 is reduced, and at the same time,hydroxide ions (OH⁻) are generated as shown in the left figure in FIG.2. It is presumed that this brings the pH into the range where theabundance ratio of hydrogen phosphate ions becomes higher and that thesehydrogen phosphate ions react with tin ions to form tin phosphate suchas Sn₃(PO₄)₂. The tin phosphate partly dissolves to form aluminumphosphate when the aluminum-oxygen compound layer 30 is formed by meansof electrolytic treatment, as described later.

In the present embodiment, when the phosphate compound layer 20 isformed, the tin-plated steel sheet 10 may be subjected to anodeelectrolytic treatment using an electrolytic treatment liquid containingphosphate ions before the cathode electrolytic treatment aforementionedis conducted. The oxide film layer formed on the surface of thetin-plated steel sheet 10 is moderately removed by anode electrolytictreatment, and thereafter, the cathode electrolytic treatment allows thetin-plating layer 12 of the tin-plated steel sheet 10 to dissolveeasily. This facilitates formation of the phosphate compound layer 20.This cathode electrolytic treatment is preferably conducted as follows:the tin-plated steel sheet 10 is immersed in an electrolytic treatmentliquid and subjected to anode electrolytic treatment; and thereafter,while the tin-plated steel sheet 10 remains immersed in the electrolytictreatment liquid, the polarities of the anode and cathode are reversedin the energization control circuit, followed by conducting the cathodeelectrolytic treatment aforementioned. This facilitates control of thesolution and also improves the working efficiency when anodeelectrolytic treatment and cathode electrolytic treatment are conducted.

In the present embodiment, when a coating layer 40 formed of an organicmaterial is formed on the surface of the surface-treated steel sheet 1,the coating layer 40 on the surface-treated steel sheet 1 has excellentadhesion due to the formation of phosphate compound layer 20 containingtin phosphate by the cathode electrolytic treatment aforementioned.Specifically, in the case of forming the aluminum-oxygen compound layer30 directly on the surface-treated steel sheet 1, if the coating layer40 is formed by baking coating or the like, an oxide film layer coveringthe tin-plating layer 12 of the surface-treated steel sheet 1 grows dueto the heat from baking, and this growth may cause the aluminum-oxygencompound layer 30 and the coating layer 40 to delaminate from this oxidefilm layer. In contrast, by forming the phosphate compound layer 20aforementioned, growth of an oxide film layer covering the tin-platinglayer 12 when the coating layer 40 is formed can be reduced, and as aresult, the adhesion of coating layer 40 formed on the surface-treatedsteel sheet 1 can be increased.

In the present embodiment, by forming phosphate compound layer 20containing tin phosphate by cathode electrolytic treatmentaforementioned, the surface-treated steel sheet 1 to be obtained willhave improved corrosion resistance. Specifically, the present inventorshave found that the tin-plated steel sheet 10 is subjected to cathodeelectrolytic treatment to cause hydrogen phosphate ions to react withtin ions to thereby form tin phosphate as aforementioned and that,accordingly, the chemical bonding state and surface morphology of thetin phosphate formed by these hydrogen phosphate ions makes the tinphosphate difficult to dissolve in an electrolytic treatment liquid usedwhen the aluminum-oxygen compound layer 30 described later is formed bymeans of electrolytic treatment. Accordingly, the corrosion resistanceof the surface-treated steel sheet 1 to be obtained can be improved. Thesurface-treated steel sheet 1 to be obtained accordingly will havesufficient corrosion resistance even when the coating layer 40 mainlyformed of an organic material is not formed on its surface and thus canbe suitably used for a metal container including no coating layer 40 inapplications requiring no coating.

In surface-treated steel sheet 1 of the present embodiment, thetin-plating layer 12 of the tin-plated steel sheet 10 partly dissolvesto form the phosphate compound layer 20 and this phosphate compoundlayer 20 partly dissolves to form the aluminum-oxygen compound layer 30,as aforementioned. Thus, the tin-plating layer 12, the phosphatecompound layer 20, and the aluminum-oxygen compound layer 30 arestructured such that they are intermixed with one another near eachboundary. For example, in the present embodiment, both the phosphatecompound layer 20 and the aluminum-oxygen compound layer 30 may containtin phosphate and aluminum phosphate.

FIG. 6 is a cross-sectional photograph and the result of quantitativeanalysis by means of energy dispersive X-ray spectrometry (EDS) at eachpoint of the phosphate compound layer 20 and the aluminum-oxygencompound layer 30 of the surface-treated steel sheet obtained in Exampleof the present invention. As aforementioned, it was confirmed that theboundary between the phosphate compound layer 20 and the aluminum-oxygencompound layer 30 is not obvious and that both the phosphate compoundlayer 20 and the aluminum-oxygen compound layer 30 may contain tinphosphate and aluminum phosphate.

As for the electrolytic treatment liquid for forming the phosphatecompound layer 20, phosphoric acid (H₃PO₄), sodium dihydrogen phosphate(NaH₂PO₄), disodium hydrogen phosphate (Na₂HPO₄), phosphorous acid(H₃PO₃) or the like can be used as a compound to form phosphate ions inthe electrolytic treatment liquid. These phosphoric acid and phosphatesmay be used singly or in admixture. Of these, a mixture of phosphoricacid and sodium dihydrogen phosphate, which enables tin phosphate todeposit satisfactorily as the phosphate compound layer 20, is suitable.

The concentration of the phosphate ions in the electrolytic treatmentliquid is not particularly limited and is preferably 5 to 200 g/L interms of phosphorus. A concentration of the phosphate ions in theelectrolytic treatment liquid within the above range enables tinphosphate to deposit satisfactorily on the tin-plated steel sheet 10.

The pH of the electrolytic treatment liquid is not particularly limitedand is preferably 1 to 7. With a pH of less than 1, the tin phosphateformed tends to dissolve. In contrast, with a pH of more than 7, theoxide film layer on the surface of the tin-plated steel sheet 10insufficiently dissolves, which makes it difficult to form the phosphatecompound layer 20 on a portion in which the oxide film layer is muchremaining, and thus it may be impossible to form the homogeneousphosphate compound layer 20 on the tin-plated steel sheet 10.

The current density when the anode electrolytic treatment or cathodeelectrolytic treatment aforementioned is conducted is not particularlylimited and is preferably 1 to 30 A/dm². A current density within theabove range enables satisfactory formation of the phosphate compoundlayer 20 on the tin-plated steel sheet 10.

When the tin-plated steel sheet 10 is subjected to anode electrolytictreatment or cathode electrolytic treatment, a counter electrode plateplaced opposite to the tin-plated steel sheet 10 may be any counterelectrode plate that does not dissolve in the electrolytic treatmentliquid while the electrolytic treatment is conducted. Because ofdifficulty to dissolve in the electrolytic treatment liquid, a titaniumplate coated with iridium oxide or a titanium plate coated with platinumis preferable.

The energizing time when anode electrolytic treatment or cathodeelectrolytic treatment is conducted is not particularly limited and ispreferably 0.15 to 3.0 seconds, more preferably 0.15 to 1.0 seconds.When cathode electrolytic treatment is conducted after anodeelectrolytic treatment as aforementioned, the energizing time of cathodeelectrolytic treatment is preferably is equivalent to the energizingtime of anode electrolytic treatment. The number of cycles of theenergizing time and stop of energization when cathode electrolytictreatment or cathode electrolysis after anode electrolytic treatment isconducted is preferably 1 to 10. The number of cycles may be adjustedtogether with the energizing time so as to achieve an appropriatephosphorous content in the phosphate compound layer 20. The appropriatephosphorous content in the phosphate compound layer 20 is preferably 0.5to 20 mg/m², more preferably 0.5 to 5.0 mg/m², particularly preferably0.7 to 4.0 mg/m².

<Aluminum-Oxygen Compound Layer 30>

In the present embodiment, the tin-plated steel sheet 10 including thephosphate compound layer 20 formed thereon is washed with water asappropriate and subsequently subjected to electrolytic treatment in anelectrolytic treatment liquid containing Al ions to allow analuminum-oxygen compound to deposit on the phosphate compound layer 20to thereby form the aluminum-oxygen compound layer 30. The electrolytictreatment method may be either anode electrolytic treatment or cathodeelectrolytic treatment. Cathode electrolytic treatment is preferable, inview of capable of satisfactorily forming the aluminum-oxygen compoundlayer 30.

The content of the Al ion in the electrolytic treatment liquid forforming the aluminum-oxygen compound layer 30 can be appropriatelyselected depending on the amount of the film of the aluminum-oxygencompound layer 30 and is preferably 0.5 to 10 g/L, more preferably 1 to5 g/L in terms of the mass concentration of the Al atom. A content ofthe Al ion in the electrolytic treatment liquid within the above rangecan improve the stability of the electrolytic treatment liquid and alsoimprove the deposition efficiency of the aluminum-oxygen compound.

In the present embodiment, nitrate ions may be added to the electrolytictreatment liquid used for forming the aluminum-oxygen compound layer 30.When nitrate ions are added to the electrolytic treatment liquid, thecontent of the nitrate ion in the electrolytic treatment liquid ispreferably 11,500 to 25,000 ppm by weight. A content of the nitrate ionwithin the above range in the electrolytic treatment liquid enablesadjustment of the electrical conductivity within a suitable range.

The electrolytic treatment liquid used for forming the aluminum-oxygencompound layer 30 is preferably free of F ions. When the electrolytictreatment liquid used for forming the aluminum-oxygen compound layer 30is free of F ions, the aluminum-oxygen compound layer 30 having a smallparticle size and being dense can be formed, and thus, the corrosionresistance of the surface-treated steel sheet 1 to be obtained can beimproved. If F ions are contained in the electrolytic treatment liquid,they cause formation of SnF₂, which is then captured in thealuminum-oxygen compound layer 30 to thereby reduce the sulfideblackening resistance and corrosion resistance.

The electrolytic treatment liquid may be any electrolytic treatmentliquid containing substantially no F ion, or may contain F ions in anamount of an impurity level approximately. In other words, F atoms areincluded at a very low level in industrial water, and thus, F ionsderived from such F atoms may be included in the electrolytic treatmentliquid. In this case, the F ions are present in a form of F ions formingcomplex ions with metal, free F ions, and the like, in the electrolytictreatment liquid. When the total amount of these F ions is preferably 50ppm by weight or less, more preferably 20 ppm by weight or less, morepreferably 5 ppm by weight or less, it can be determined that the amountof F ions contained in the electrolytic treatment liquid isapproximately an impurity level, i.e., that the electrolytic treatmentliquid contains substantially no F ion.

In the present invention, as for the method to measure the content of Fions and nitrate ions in the electrolytic treatment liquid, for example,a method of measurement by quantitative analysis using an ionchromatography can be used.

In addition, to the electrolytic treatment liquid for forming thealuminum-oxygen compound layer 30, at least one or more of additivesselected from organic acids (citric acid, lactic acid, tartaric acid,glycolic acid and the like), polyacrylic acid, polyitaconic acid, phenolresin and the like may be added. In the present embodiment,appropriately adding one of these additives singly or two or more ofthese in combination to this electrolytic treatment liquid enables thealuminum-oxygen compound layer 30 to be formed to contain an organicmaterial. This can improve the adhesion of the coating layer 40 to beformed on the aluminum-oxygen compound layer 30.

In addition, the content of the phosphate ion in the electrolytictreatment liquid for forming the aluminum-oxygen compound layer 30 isdesirably adjusted. The content of phosphate ion in the electrolytictreatment liquid is preferably 0.55 g/L or less, more preferably 0.33g/L or less, more preferably 0.11 g/L in terms of phosphorus.

Specifically, in the present embodiment, when the aluminum-oxygencompound layer 30 is formed by means of electrolytic treatment, tinphosphate and the like dissolve from the phosphate compound layer 20into the electrolytic treatment liquid used for forming thealuminum-oxygen compound layer 30, and phosphate ions are generated.With an unduly large amount of the phosphate ions, the phosphate ionsbind to Al ions to precipitate as aluminum phosphate in the electrolytictreatment liquid. This leads to decrease in the amount of Al ions in theelectrolytic treatment liquid used for forming the aluminum-oxygencompound layer 30 to thereby reduce the efficiency of forming thealuminum-oxygen compound layer 30. Moreover, the aluminum-oxygencompound layer 30 to be formed by precipitation of aluminum phosphate inthe electrolytic treatment liquid becomes inhomogeneous and exhibitsspots, which tend to reduce the quality of appearance although withoutany quality problems.

In contrast, a content of phosphate ions in the electrolytic treatmentliquid for forming the aluminum-oxygen compound layer 30 within theabove range makes the aluminum-oxygen compound layer 30 to be formedhomogeneous and improves the quality of appearance of thesurface-treated steel sheet 1 to be obtained.

When the aluminum-oxygen compound layer 30 is formed by means ofelectrolytic treatment, an intermittent electrolysis method where acycle of energization and stop of energization is repeated is preferablyused. When the method is used, the total energization time for the basematerial (the total energization time when the cycle of energization andstop of energization is repeated for several times) is preferably 1.5seconds or less, more preferably 1 second or less. The number of cyclesof energization and stop of energization is preferably 1 to 10 and maybe adjusted together with the energizing time so as to achieve anappropriate content of aluminum in the aluminum-oxygen compound layer30. The appropriate content of aluminum in the aluminum-oxygen compoundlayer 30 is preferably 3 to 40 mg/m², more preferably 5 to 15 mg/m²,particularly preferably 5.1 to 10.6 mg/m².

When the aluminum-oxygen compound layer 30 is formed, a counterelectrode plate placed opposite to the base material may be any counterelectrode plate that does not dissolve in the electrolytic treatmentliquid while the electrolytic treatment is conducted. In view ofdifficulty to dissolve in the electrolytic treatment liquid due to smalloxygen overvoltage, a titanium plate coated with iridium oxide or atitanium plate coated with platinum is preferable.

The aluminum-oxygen compound layer 30 formed as described above ismainly formed of aluminum oxide and the like and also contains aluminumhydroxide and phosphates. Example of the phosphate include aluminumphosphate and oxygen compounds containing phosphoric acid (such asAl(PO₄)_(y)O_(z)). This phosphate precipitates as the aluminum-oxygencompound layer 30 as follows. Specifically, in the present embodiment,when the tin-plated steel sheet 10 including the phosphate compoundlayer 20 formed thereon is subjected to electrolytic treatment with anelectrolytic treatment liquid containing Al ions, as aforementioned, aportion of tin phosphate forming the phosphate compound layer 20dissolves, and phosphate ions generated by this dissolution causephosphates such as aluminum phosphate and oxygen compounds containingphosphoric acid to precipitate. Furthermore, in formation of thealuminum-oxygen compound layer 30, dissolution of the phosphate compoundlayer 20 or dissolution of a portion which is not coated the phosphatecompound and thus from which tin plating is exposed leads to productionof tin ions Sn²⁺ in the electrolytic treatment liquid. Thus, it ispresumed that, in addition to tin phosphate, tin oxide (SnO_(x)) ispartly contained in the aluminum-oxygen compound layer 30. According tothe present embodiment, allowing the aluminum-oxygen compound layer 30to contain phosphate can reduce growth of the oxide film layer of thetin-plated steel sheet 10 due to the heat from baking when the coatinglayer 40 is formed on the aluminum-oxygen compound layer 30 by bakingcoating. As a result, the adhesion of the coating layer 40 to be formedon the surface-treated steel sheet 1 can be improved.

The reason why such an effect can be achieved by allowing thealuminum-oxygen compound layer 30 to contain phosphate is notnecessarily obvious but can be inferred as follows. First, as describedabove, in the phosphate compound layer 20 obtained by means of cathodeelectrolytic treatment, hydrogen phosphate ions mainly react with tinions to form tin phosphate. Accordingly, the chemical bonding state andsurface morphology of the tin phosphate formed by these hydrogenphosphate ions make the aluminum-oxygen compound layer 30 difficult todissolve in an electrolytic treatment liquid used when thealuminum-oxygen compound layer 30 is formed by means of electrolytictreatment. Then, growth of an oxide film layer of the tin-plated steelsheet 10 due to the heat from baking can be reduced. As a result, theadhesion of the coating layer 40 to be formed on the surface-treatedsteel sheet 1 is improved. When only tin phosphate is formed on thetin-plated steel sheet 10, the following is presumed: the tin phosphatecoating film deteriorates over time; therefore, though increase in oxidefilm in the coating and baking steps can be reduced in the initialstage, the coating film gradually becomes fragile; and thus, theadhesion to the coating layer 40 is reduced. In the present invention,it is presumed that deterioration of tin phosphate is reduced and thusthe adhesion to the coating layer 40 becomes satisfactory by providingthe aluminum-oxygen compound layer 30.

In the aluminum-oxygen compound layer 30, when the 3 d_(5/2) spectrum oftin in the aluminum-oxygen compound layer is determined using an X-rayphotoelectron spectroscopy, the ratio of the integration value of theprofile derived from tin oxide to the integration value of the profilederived from tin phosphate (a value obtained by integrating the spectrumintensity by the binding energy) (tin oxide/tin phosphate) is 6.9 ormore. As shown in FIG. 3, the peak of the profile derived from tinphosphate is observed at around 489.0 eV and the peak of the profilederived from tin oxide is observed at around 487.5 eV. The peak ataround 485.0 eV seems to be derived from metal tin. Tin oxide containsSnO and SnO₂, which were not separated. Thus, these were handled as onepeak. FIG. 3 herein is a graph showing a spectrum obtained by measuringthe aluminum-oxygen compound layer 30 of the surface-treated steel sheet1 of Example 6 described later by an X-ray photoelectron spectroscopy.The vertical axis represents the spectrum intensity and the horizontalaxis represents the binding energy (eV). In the present embodiment,setting the ratio of tin oxide/tin phosphate aforementioned within theabove range allows the surface-treated steel sheet 1 to be obtained tohave excellent corrosion resistance as well as to have excellentadhesion of the coating layer 40 to be formed on the surface.

The content of aluminum in the aluminum-oxygen compound layer 30 ispreferably form 5 to 15 mg/m², more preferably 5.1 to 10.6 mg/m². Anunduly low content of aluminum in the aluminum-oxygen compound layer 30leads to increase in the oxide film layer on the surface of thetin-plated steel sheet 10 when the coating layer 40 formed of an organicmaterial is formed by baking coating. Thus, the aluminum-oxygen compoundlayer 30 and the coating layer 40 tend to easily delaminate from theoxide film layer. In contrast, an unduly high content of aluminum in thealuminum-oxygen compound layer 30 may make the aluminum-oxygen compoundlayer 30 brittle to lead to cohesive failure.

The aluminum-oxygen compound layer 30 contain phosphate asaforementioned. The content ratio of the amount of phosphorus (mol/m²)to the amount of aluminum (mol/m²) in the aluminum-oxygen compound layer30 (P/Al) is preferably 0.06 to 0.35, more preferably 0.07 to 0.27. Whenthe above content ratio (P/Al) is less than 0.06, the oxide film layeron the surface of the tin-plated steel sheet 10 grows due to the heatfrom baking when the coating layer 40 formed of an organic material isformed by baking coating. Thus, the aluminum-oxygen compound layer 30and the coating layer 40 tend to easily delaminate from the oxide filmlayer. In contrast, when the above content ratio (P/Al) is more than0.35, the aluminum-oxygen compound layer 30 to be formed becomesinhomogeneous and exhibits spots, which tend to reduce the quality ofappearance although without any quality problems.

The surface-treated steel sheet 1 of the present embodiment is obtainedin the above manner.

In the surface-treated steel sheet 1 of the present embodiment, thetotal amount of phosphorus contained in each of layers to be formed onthe steel sheet 11 (tin-plating layer 12, phosphate compound layer 20,and aluminum-oxygen compound layer 30) is preferably 0.5 to 10 mg/m²,more preferably 0.7 to 2.5 mg/m². When the total amount of phosphoruscontained in each of the layers is unduly low, the oxide film layer onthe tin-plated steel sheet 10 grows due to the heat from baking when thecoating layer 40 formed of an organic material is formed by bakingcoating. Thus, the aluminum-oxygen compound layer 30 and the coatinglayer 40 tend to easily delaminate from the oxide film layer. Incontrast, when the total amount of phosphorus contained in each of thelayer is unduly high, the content proportion of tin phosphate increasesin phosphate compound layer 20, and this tin phosphate serves as aninsulator. Thus, an aluminum-oxygen compound inhomogeneouslyprecipitates during the electrolytic treatment forming thealuminum-oxygen compound layer 30, and the aluminum-oxygen compoundlayer 30 to be formed exhibits spots, which tend to reduce the qualityof appearance although without any quality problems.

In the present embodiment, an example of the method for measuring thetotal amount of phosphorus contained in each of layers formed on thesteel sheet 11 include a method involving quantitatively analyzing thesurface-treated steel sheet 1 using an X-ray fluorescence spectrometer.

<Metal Container>

The surface-treated steel sheet 1 of the present embodiment may be usedas, but not particularly limited to, a member such as a can container orcan lid. When the surface-treated steel sheet 1 is used as a member suchas a can container or can lid, the surface-treated steel sheet 1 may beused as is (used in applications requiring no coating, in which nocoating layer 40 is formed on the surface) to be shaped into anon-coated can container or can lid. After a coating layer 40 formed ofan organic material is formed on the aluminum-oxygen compound layer 30of the surface-treated steel sheet 1, the steel sheet 1 may be shapedinto a can container or can lid. The organic material forming thecoating layer 40 is not particularly limited and may be selectedappropriately depending on the usage of the surface-treated steel sheet1 (for example, a usage of can containers to be filled with a specificcontent). Examples thereof include thermoplastic resins and thermosetresins.

As the thermoplastic resin, olefinic resin films such as polyethylene,polypropylene, ethylene-propylene copolymers, ethylene-vinyl acetatecopolymers, ethylene-acryl ester copolymers, and ionomers, polyesterfilms such as polyethylene terephthalate and polybutylene terephthalate,unstretched film or biaxially oriented films such as polyvinyl chloridefilms and polyvinylidene chloride films, or polyamide films such asnylon 6, nylon 6,6, nylon 11, and nylon 12 or the like can be used. Ofthese, unoriented polyethylene terephthalate prepared by copolymerizingwith isophtalic acid is particularly preferable. Alternatively, such anorganic material for forming the coating layer 40 may be used singly ormay be blended with a different organic material.

As the thermoset resin, epoxy-phenol resins, polyester resins or thelike can be used.

In the case of coating with a thermoplastic resin as the coating layer40, the layer may be a single resin layer or may be a multi-layeredresin layer prepared by coextrusion or the like. Using a multi-layeredpolyester resin layer is advantageous for the following reason: apolyester resin having a composition excellent in the adhesion propertycan be selected as the material of an underlying layer located at theside of the surface-treated steel sheet 1, and a polyester resin of acomposition excellent in the resistance to the contents of cans, i.e.,resistance to extraction and non-adsorptive property for flavorconstituents, can be selected as the material for a surface layer.

Examples of the multi-layered polyester resin layer include, indicatedas surface layer/lower layer, polyethylene terephthalate/polyethyleneterephthalate·isophthalate, polyethyleneterephthalate/polyethylene·cyclohexylene dimethylene·terephthalate,polyethylene terephthalate·isophthalate having a low isophthalatecontent/polyethylene terephthalate·isophthalate having a highisophthalate content, and polyethyleneterephthalate·isophthalate/[blended product of polyethyleneterephthalate·isophthalate and polybuthylene terephthalate·adipate], ofcourse, without limited to limitation to the above examples. Thethickness ratio of surface layer:lower layer is desirably in the rangeof 5:95 to 95:5.

To the above coating layer 40, compounding agents for resins known perse, for example, an antiblocking agent such as amorphous silica, aninorganic filler, various antistatic agents, a slip agent, anantioxidant (such as tocopherol), an ultraviolet absorbent and the likecan be compounded in accordance with known formulations.

It is desired that the thickness of the coating layer 40 to be formed onthe surface-treated steel sheet 1 obtained by the present invention begenerally in the range of 3 to 50 μm, particularly of 5 to 40 μm in thecase of thermoplastic resin coating. In the case of a coating layer, itis preferred that the thickness after backing be in the range of 1 to 50μm, particularly of 3 to 30 μm. If the thickness is below the aboverange, the corrosion resistance will be insufficient, while if thethickness is above the above range, problems in formability may arise.

Thermal bonding of a polyester resin to the surface-treated steel sheet1 is conducted using the quantity of heat held by the molten-resin layerand the quantity of heat held by the surface-treated steel sheet 1. Theheating temperature of the surface-treated steel sheet 1 isappropriately 90° C. to 290° C. in general, particularly 100° C. to 230°C., whereas the temperature of the laminating rolls is appropriately inthe range of 10° C. to 150° C.

Furthermore, the coating layer 40 to be formed on the surface-treatedsteel sheet 1 can be also formed by thermally bonding a polyester resinfilm made in advance with the T-die method or inflation film-formationmethod to the surface-treated steel sheet 1. As for the film, anunstretched film prepared with the cast molding method in which theextruded film is immediately cooled can also be used. Also, abiaxially-stretched film obtained by biaxially stretching this film at astretching temperature, either subsequently or simultaneously, andthermally fixing the film after stretching can also be used.

On the surface of the surface-treated steel sheet 1 of the presentinvention, for example, the coating layer 40 is formed to obtain anorganic material-coated steel sheet, which then can be processed andshaped into a can container. Examples of the can container include, butnot particularly limited to, a seamless can 5 (two-piece can) shown inFIG. 4(A) and a three-piece can 5 a (welded can) shown in FIG. 4(B). Abody 51 and an upper lid 52 constituting the seamless can 5 and a body51 a, an upper lid 52 a, and a lower lid 53 constituting the three-piececan 5 a are all formed using the organic material-coated steel sheetobtained by forming the coating layer 40 on the surface-treated steelsheet 1 of the present embodiment. In FIGS. 4(A) and 4(B), thecross-sectional views of the seamless can 5 and the three-piece can 5 aare views obtained by rotating FIG. 1 above by 90° such that the coatinglayer 40 is located inside the can. The cans 5 and 5 a respectivelyshown in FIGS. 4(A) and 4(B) may be produced such that the coating layer40 is located inside the can by any conventionally known means, such asdrawing process, drawing/redrawing process, stretching process viadrawing/redrawing, stretching/ironing process via drawing/redrawing, ordrawing/ironing process.

The seamless can 5, which is subjected to a highly sophisticatedprocess, such as stretching process via drawing/redrawing andstretching/ironing process via drawing/redrawing, preferably has thecoating layer 40 formed of thermoplastic resin coating by an extrusioncoating method.

That is, the organic material-coated steel sheet, having excellentprocessing adhesion, has excellent coating adhesion even when subjectedto severe processing, and is capable of provide a seamless can havingexcellent corrosion resistance.

On the surface of the surface-treated steel sheet 1 of the presentinvention, for example, the coating layer 40 is formed to obtain anorganic material-coated steel sheet as aforementioned, which then can beprocessed and produced into a can lid. Examples of the can lid include,but not particularly limited to, flat lids, stay-on-tub type easy-opencan lids, and pull-open type easy-open can lids.

EXAMPLES

Hereinafter, the present invention will be specifically described withreference to examples, but the present invention is not limited to theseexamples.

Evaluation methods of each property are as follows.

<Analysis of Electrolytic Treatment Liquid>

As for an electrolytic treatment liquid, the phosphate ion concentrationor Al ion concentration was measured using an ICP emission spectrometer(manufactured by SHIMADZU CORPORATION, ICPE-9000), and the F ionconcentration and nitrate ion concentration were measured with an ionchromatograph (manufactured by Dionex Corporation, DX-500). The pH ofthe above electrolytic treatment liquid was also measured using a pHmeter (manufactured by HORIBA, Ltd.).

<Measurement of Amount of Phosphorus and Amount of Aluminum>

As for the surface-treated steel sheet 1, the amount of phosphorus andthe amount of aluminum contained in each of the layers on the steelsheet 11 were measured in the unit of mg/m² using an X-ray fluorescencespectrometer (manufactured by Rigaku Corporation, ZSX100e). In addition,the content ratio of the amount of phosphorus (mol/m²) to the amount ofaluminum (mol/m²) (P/Al) was calculated by converting the measurementvalues obtained into the unit of mol/m². Measurement of the amount ofphosphorus and the amount of aluminum and calculation of P/Al wereconducted on all the Examples and Comparative Examples described later.

<Measurement of Tin Compound in Aluminum-Oxygen Compound Layer 30>

As for the surface-treated steel sheet 1, the 3 d_(5/2) spectrum of tinwas measured using an X-ray photoelectron spectroscopy under thefollowing conditions to thereby determine the ratio of the integrationvalue of the profile derived from tin oxide to the integration value ofthe profile derived from tin phosphate (tin oxide/tin phosphate) in thealuminum-oxygen compound layer 30. Measurement of the tin compound inthe aluminum-oxygen compound layer 30 and calculation of the above ratio(tin oxide/tin phosphate) were conducted only on Examples 6 describedlater.

The 3 d_(5/2) spectrum of tin obtained was analyzed by waveformseparation using software.

Measurement apparatus: JPS-9200 manufactured by JEOL Ltd.

Excited X-ray source: MgKα voltage 12 kV, current 25 mA

Measurement diameter: diameter 3 mm

Photoelectron take-off angle: 90° (0° with respect to the normal of thesample)

Analysis software: SpecSurf (ver. 1.7.3.9) manufactured by JEOL Ltd.

Waveform separation conditions: binding energy of tin oxide 487.5 eV,binding energy of tin phosphate 489.1 eV, and binding energy of metaltin 485.4 eV

<Cross-Section Observation and Quantitative Analysis of Surface-TreatedSteel Sheet>

The surface-treated steel sheet 1 was subjected to carbon vapordeposition, and then further deposited with carbon to a thickness ofabout 1 μm in an FIB apparatus. A sample was cut out by a microsamplingmethod and fixed on a copper support. Thereafter, a cross-sectional TEMspecimen was prepared by FIB processing and quantitatively analyzed byconducting TEM observation and EDS analysis on each point.

<FIB> FB-2000C-model focused ion beam apparatus manufactured by Hitachi,Ltd., accelerating voltage 40 kV

<TEM> JEM-2010F-model field emission transmission electron microscopemanufactured by JEOL Ltd., accelerating voltage 200 kV

<EDS> UTW-type Si (Li) semiconductor detector manufactured by NORANInstruments, Inc., analysis area 1 nm

Cross-section observation and quantitative analysis of thesurface-treated steel sheet were conducted only on Example 6 describedlater.

<Coating Adhesion Evaluation>

The organic material-coated steel sheet obtained by forming the coatinglayer 40 on the surface-treated steel sheet 1 was subjected to retorttreatment at a temperature of 125° C. for 30 minutes. A grid having aspacing of 5 mm and a depth reaching the steel sheet 11 was formed onthe steel sheet, and the coating layer was peeled off with tape. Thedegree of peeling was visually observed and evaluated based on thefollowing criteria.

Coating adhesion evaluation was conducted on all the Examples andComparative Examples described later.

Score 3: As a determination result of visual observation, no peeling ofthe coating was observed.

Score 2: As a determination result of visual observation, peeling of thecoating was observed at an area ratio of ⅕ or less.

Score 1: As a determination result of visual observation, peeling of thecoating was observed at an area ratio of more than ⅕.

In the coating adhesion evaluation, it was determined that, in the casewhere the score was 2 or more in the above criteria, the surface-treatedsteel sheet 1 had sufficient coating adhesion for applications of cansfor food and beverages.

<Evaluation of Growth of Oxide Film Layer>

The surface-treated steel sheet 1 was thermally treated at a temperatureof 205° C. for 30 minutes. The amount of the oxide film layer of tinformed on the surface of tin-plated steel sheet 10 was determined bothbefore and after the thermal treatment. The amount of electricityrequired to remove the oxide film layer by electrochemical reduction wastaken as the amount of the oxide film layer was determined. A 1/1000 Nhydrogen bromide solution was used as the electrolytic liquid, andelectrolysis was conducted under a condition of a current density of 25μA/cm². Growth of the oxide film layer was evaluated by a value obtainedby dividing the amount of the oxide film layer after the thermaltreatment by the amount of the oxide film layer before the thermaltreatment (oxide film layer after the thermal treatment/oxide film layerbefore the thermal treatment). It was considered that the oxide filmlayer is likely to grow by the thermal treatment as the value is larger.Evaluation of growth of the oxide film layer was carried out on all theExamples and Comparative Examples described later.

∘: Oxide film layer after thermal treatment/oxide film layer beforethermal treatment was 1.2 or less.

Δ: Oxide film layer after thermal treatment/oxide film layer beforethermal treatment was more than 1.2 and 1.4 or less.

×: Oxide film layer after thermal treatment/oxide film layer beforethermal treatment exceeded 1.4.

<Corrosion Resistance Evaluation (Model Solution)>

Test pieces were prepared by cutting the organic material-coated steelsheet obtained by forming the coating layer 40 on the surface-treatedsteel sheet 1 into 40-mm square pieces and protecting the cut face with3-mm wide tape. Subsequently, a crosscut scratch having a depth reachingthe steel sheet was made on the test piece prepared using a cutter. Thetest piece was subjected to 3 mm bulging process by an Erichsen tester(manufactured by Coating Tester Co., Ltd.) such that the intersection ofthe crosscut would be the apex of the bulging-processed portion. Then,the test piece bulging-processed was placed in a sealed container. Afterthe container was filled with the following model solution and storedunder an environment of 90° C. for 24 hours.

Model solution: an aqueous solution containing NaCl and citric acid eachdissolved at 1.5% by weight

Thereafter, the sealed container was opened. The degree of corrosion ofthe test piece was visually observed and evaluated based on thefollowing criteria. Corrosion resistance evaluation (model solution) wasconducted on all the Examples and Comparative Examples described later.

Score 3: As a determination result of visual observation, the degree ofcorrosion was obviously lower than that of Comparative Example 2.

Score 2: As a determination result of visual observation, the degree ofcorrosion was equivalent to that of Comparative Example 2.

Score 1: As a determination result of visual observation, the degree ofcorrosion was obviously higher than that of Comparative Example 2.

In the corrosion resistance evaluation (model solution), it wasdetermined that, in the case where the score was 2 or more in the abovecriteria, the surface-treated steel sheet 1 had sufficient corrosionresistance for applications of cans for food and beverages.

<Detinning Evaluation (Model Solution)>

Test pieces were prepared by cutting the surface-treated steel sheet 1into a disc having a diameter of 49 mm and protecting the cut face with3-mm wide tape. The amount of Sn in the test pieces prepared wasmeasured with X-ray fluorescence. Then, the test piece was placed in asealable container. After the container was filled with the followingmodel solution and stored under an environment of 37° C. for 10 days.

Model solution: an aqueous solution containing 1% by weight of aceticacid and 10% by weight of sucrose dissolved

Thereafter, the sealed container was opened. The amount of Sn in thetest piece after storage under an environment of 37° C. for 10 days wasmeasured with X-ray fluorescence, and the % of the amount of Snremaining was calculated using the following formula.

Amount of Sn remaining (%)=(Amount of Sn before the lapse of thetime−Amount of Sn after the lapse of the time)/Amount of Sn before thelapse of the time×100

When the % of the amount of Sn remaining was equivalent to or higherthan that of Reference Example 1, it was determined that thesurface-treated steel sheet 1 had sufficient detinning for applicationsof cans for food and beverages. The detinning evaluation (modelsolution) was conducted on Example 8, Comparative Example 3, andReference Example 1 described later.

Example 1

First, a low carbon cold-rolled sheet (thickness: 0.225 mm) was providedas a steel sheet 11.

Subsequently, the steel sheet provided was degreased by subjecting thesteel sheet to cathode electrolytic treatment using an aqueous solutionof an alkaline degreasing agent (manufactured by Nippon Quaker Chemical,Ltd., Formula 618-TK2) under conditions of 60° C. and 10 seconds. Afterwashed with tap water, the steel sheet degreased was acid pickled byimmersion in an acid pickling agent (an aqueous solution of 5% by volumeof sulfuric acid) at normal temperature for five seconds. Thereafter,the steel sheet was washed with tap water and subjected to tin platingusing a known ferrostan bath under the following conditions to form atin-plating layer 12 having an amount of tin of 2.8 g/m² on the surfaceof the steel sheet. Subsequently, the steel sheet including thetin-plating layer 12 formed was washed with water and made to generateheat by supplying a direct current thereto. The steel sheet wassubjected to reflow treatment, in which the steel sheet was heated to atemperature equal to or higher than the melting temperature of tin andthen rapidly cooled by sprinkling tap water thereon, to thereby preparea tin-plated steel sheet 10.

Bath temperature: 40° C.

Current density: 10 A/dm²

Anode material: commercially available 99.999% metal tin

Total energizing time: 5 seconds (number of cycles: 5, when one cycleincluded energizing time of 1 second and stop time of 0.5 seconds)

Then, the tin-plated steel sheet 10 obtained was immersed in anelectrolytic treatment liquid with stirring and subjected to cathodeelectrolytic treatment under the following conditions using an iridiumoxide-coated titanium plate positioned at an interelectrode distance of17 mm as the cathode to thereby form a phosphate compound layer 20 onthe tin-plated steel sheet 10.

Electrolytic treatment liquid: an aqueous solution having a pH of 1.3containing phosphorous acid dissolved at a concentration of 10 g/L(treatment liquid A in Table 1)

Temperature of the electrolytic treatment liquid: 40° C.

Current density: 3 A/dm²

Total energizing time: 0.5 seconds (energizing time 0.5 seconds, numberof cycles 1)

The electrolytic treatment liquid used for forming the phosphatecompound layer 20 was analyzed in accordance with the methodaforementioned. The result is listed in Table 1. Table 1 also shows theconcentration of phosphorous atoms (g/L) calculated depending on theconcentration of the phosphate compound dissolved. In Example 1, theelectrolytic treatment liquid shown as the treatment solution A in Table1 was used for forming the phosphate compound layer 20. Likewise, atreatment liquid B was used in Examples 2 and 3 described later, and atreatment liquid C was used in Examples 4 to 8 and Comparative Example 1described later. Additionally, the conditions for the electrolytictreatment when the phosphate compound layer 20 was formed on thetin-plated steel sheet 10 are shown in Table 2.

TABLE 1 Amount dis

 (g/L) Sodium Disodium Concentration (g/L) Phosphoric dihydrogenhydrogen

Al Nitrate F acid phosphate phosphate acid ion ion ion

pH Treatment liquid A 10 3.2 1.3 Treatment liquid B 3

30 17.2 1.

Treatment liquid C 10 30 10.9 2.4 Treatment liquid

indicates data missing or illegible when filed

TABLE 2 Electrolytic treatment for forming

Electrolytic treatment for forming

-oxygen compound layer compound layer Electrolysis conditionsElectrolysis conditions Total emerging Total emerging Total emergingCurrent time of

time of

Current time of

Liquid density Number of treatment treatment Liquid density Number oftreatment composition (

)

(sec) (sec) composition (

)

(sec) Example 1 A 3 1 Not applicable

4 1 0.

Example 2 B 3 1 Not applicable

4 1 0.

Example 3 B 2 1 Not applicable

4 1 0.

Example 4 C 3 1 Not applicable

4 1 0.

Example 5 C

1 Not applicable

4 1 0.

Example 6 C

1 0.

4 1 0.

Example 7 C

1 0.

2 0.

Example 8 C

1 0.

4 1 0.

Comparative C

1 Not applicable

— — — — Example 1 Comparative — — — — —

4 1 0.

Example 2 Comparative — — — — —

4 1 0.

Example 3

indicates data missing or illegible when filed

Subsequently, the tin-plated steel sheet 10 including the phosphatecompound layer 20 formed thereon was washed with water. Then, the steelsheet 10 was immersed in an electrolytic treatment liquid and subjectedto cathode electrolytic treatment with stirring under the followingconditions using an iridium oxide-coated titanium plate positioned at aninterelectrode distance of 17 mm as the anode to thereby form analuminum-oxygen compound layer 30. Thereafter, immediate water washingwith flowing water and drying provided the surface-treated steel sheet 1in which the phosphate compound layer 20 and the aluminum-oxygencompound layer 30 were formed on the tin-plated steel sheet 10 in thisorder. The electrolytic treatment liquid used for forming thealuminum-oxygen compound layer 30 was analyzed in accordance with themethod aforementioned. The result is listed in Table 1. In all theExamples and Comparative Examples except Comparative Example 1, theelectrolytic treatment liquid shown as the treatment liquid E in Table 1was used for forming the aluminum-oxygen compound layer 30. In addition,the conditions for the electrolytic treatment when the aluminum-oxygencompound layer 30 was formed are shown in Table 2.

Electrolytic treatment liquid: an aqueous solution having a pH of 3.0containing aluminum nitrate dissolved as an Al compound and having an Alion concentration of 1,500 ppm by weight, a nitrate ion concentration of15,300 ppm by weight, and a F ion concentration of 0 ppm by weight(treatment liquid E in Table 1)

Temperature of the electrolytic treatment liquid: 40° C.

Current density: 4 A/dm²

-   -   Total energizing time: 0.3 seconds (energizing time 0.3 seconds,        number of cycles: 1)

The surface-treated steel sheet 1 obtained was subjected to measurementof the amount of phosphorus and the amount of aluminum in accordancewith the method aforementioned. The result is listed in Table 3.

After thermal treatment at a temperature of 190° C. for 10 minutes andthe surface-treated steel sheet 1 was coated with epoxy phenolic paintsuch that the thickness of the coating layer after baking drying reached70 mg/dm² and baked at a temperature of 200° C. for 10 minutes tothereby obtain an organic material-coated steel sheet obtained byforming a coating layer 40 on the surface-treated steel sheet 1.Subsequently, the coating adhesion evaluation, corrosion resistanceevaluation (model solution), and evaluation of growth of the oxide filmlayer were conducted on the organic material-coated steel sheet obtainedin accordance with the method aforementioned. The result is listed inTable 3.

Example 2

The surface-treated steel sheet 1 and organic material-coated steelsheet were prepared and evaluated in the same manner as in Example 1except that an aqueous solution having a pH of 1.8 containing phosphoricacid at a concentration of 30 g/L and sodium dihydrogen phosphate at aconcentration of 30 g/L each dissolved (treatment liquid B in Table 1)was used as the electrolytic treatment liquid when the phosphatecompound layer 20 was formed on the tin-plated steel sheet 10. Theresult is listed in Table 3.

Example 3

The surface-treated steel sheet 1 and organic material-coated steelsheet were prepared and evaluated in the same manner as in Example 2except that the energizing time of the electrolytic treatment was set to0.3 seconds and the current density was set to 2 A/dm² when thephosphate compound layer 20 was formed and the energizing time of theelectrolytic treatment was set to 0.15 seconds when the aluminum-oxygencompound layer 30 was formed.

Example 4

The surface-treated steel sheet 1 and organic material-coated steelsheet were prepared and evaluated in the same manner as in Example 1except that an aqueous solution having a pH of 2.4 containing phosphoricacid at a concentration of 10 g/L and sodium dihydrogen phosphate at aconcentration of 30 g/L each dissolved (treatment liquid C in Table 1)was used as the electrolytic treatment liquid when the phosphatecompound layer 20 was formed on the tin-plated steel sheet 10 and thatthe energizing time of the electrolytic treatment was set to 0.15seconds when the aluminum-oxygen compound layer 30 was formed. Theresult is listed in Table 3.

Example 5

The surface-treated steel sheet 1 and organic material-coated steelsheet were prepared and evaluated in the same manner as in Example 4except that the treatment liquid C in Table 1 was used as theelectrolytic treatment liquid when the phosphate compound layer 20 wasformed on the tin-plated steel sheet 10 and the energizing time of theelectrolytic treatment was set to 0.3 seconds when the aluminum-oxygencompound layer 30 was formed. The result is listed in Table 3. As forExample 5, the tin compound in the aluminum-oxygen compound layer 30 wasmeasured, and the spectrum obtained when the tin compound in thealuminum-oxygen compound layer 30 was measured using an X-rayphotoelectron spectroscopy is shown in FIG. 5(A).

Example 6

Cathode electrolytic treatment was conducted under the same conditionsexcept that treatment in which only the polarities were reversed (anodeelectrolytic treatment) was conducted before the cathode electrolytictreatment aforementioned was conducted when the phosphate compound layer20 was formed on the tin-plated steel sheet 10. Except for this, thesurface-treated steel sheet 1 and organic material-coated steel sheetwere prepared and evaluated in the same manner as in Example 5. Theresult is listed in Table 3. As for Example 6, the tin compound in thealuminum-oxygen compound layer 30 was measured, and the spectrumobtained when the tin compound in the aluminum-oxygen compound layer 30was measured using an X-ray photoelectron spectroscopy is shown in FIG.5(B), and the TEM image and quantitative analysis results obtained bycross-section observation of the surface-treated steel sheet are shownin FIG. 6.

Example 7

The surface-treated steel sheet 1 and organic material-coated steelsheet were prepared and evaluated in the same manner as in Example 6except that the current density of the electrolytic treatment when thephosphate compound layer 20 was formed on the tin-plated steel sheet 10and the current density of the electrolytic treatment, energizing time,and number of cycles when the aluminum-oxygen compound layer 30 wasformed were changed as listed in Table 2. Before the phosphate compoundlayer 20 was formed, as pretreatment, a tin oxide film formed on thesurface of the tin-plated steel sheet was removed by immersing the steelsheet in a hydrochloric acid aqueous solution. The result is listed inTable 3.

Comparative Example 1

Coating adhesion evaluation, evaluation of growth of the oxide filmlayer, and corrosion resistance evaluation (model solution) wereconducted on the tin-plated steel sheet 10 prepared in Example 1 withoutforming the aluminum-oxygen compound layer 30 after the phosphatecompound layer 20 had been formed. The result is listed in Table 3.

Comparative Example 2

A surface-treated steel sheet was obtained by forming thealuminum-oxygen compound layer 30 in the same manner as in Example 1directly on the tin-plated steel sheet 10 prepared in Example 1 withoutforming the phosphate compound layer 20. Subsequently, the coatingadhesion evaluation, evaluation of growth of the oxide film layer, andcorrosion resistance evaluation (model solution) were conducted on thesurface-treated steel sheet obtained. The result is listed in Table 3.

TABLE 3 Properties of surface-treated steel sheet Growth of oxide filmlayer Surface-treated steel sheet After thermal Amount of Amount ofAmount of Amount of treatment/ phosphorus

P/

 oxide Coating Corrosion before thermal (mg/m

) (mg/m

) (mmol/

) (mmol/

) A

adhesion resistance treatment Evaluation Example 1 0.

0.

0.

0.07 —

3 1.1 ∘ Example 2 0.

0.

0.

0.08 —

3 1.1 ∘ Example 3 0.

0.

0.

0.32 —

3 1.1 ∘ Example 4 0.

0.

0.

0.13 —

1.2 ∘ Example 5 0.

8.4 0.

0.

0.10

1.2 ∘ Example 6 0.

10.0  0.

0.

0.07

1.2 ∘ Example 7 2.5 8.2 0.

0.

0.27 —

1.2 ∘ Comparative 0.7 — 0.

— — —

1.8 x Example 1 Comparative — 0.2 — 0.34 — — 1 Reference 1.4 Δ Example 2

indicates data missing or illegible when filed

DISCUSSION

As shown in Table 3, it was confirmed that Examples 1 to 7, in which thetin-plated steel sheet 10 was subjected to cathode electrolytictreatment to form the phosphate compound layer 20 and thealuminum-oxygen compound layer 30 was formed on this phosphate compoundlayer 20, were satisfactory in all of the results of the coatingadhesion evaluation, evaluation of growth of the oxide film layer, andcorrosion resistance evaluation (model solution), had excellentcorrosion resistance and adhesion of the coating layer 40, and weresuitable for applications such as metal containers and the like used fora prolonged period.

In contrast, as listed in Table 3, Comparative Example 1, in which thealuminum-oxygen compound layer 30 had not been formed, hadunsatisfactory results of the evaluation of growth of the oxide filmlayer. Accordingly, it is presumed that adhesion of the coating layer 40may decrease due to growth of the oxide film layer. Additionally, it wasconfirmed that Comparative Example 2, in which the aluminum-oxygencompound layer 30 was formed directly on the tin-plated steel sheet 10without forming the phosphate compound layer 20, had results of thecoating adhesion evaluation and evaluation of growth of the oxide filmlayer both worse than those of Examples and had poor adhesion of thecoating layer 40.

Example 8

The surface-treated steel sheet 1 was prepared in the same manner as inExample 1 except that the total energizing time was set to 7.5 secondsand the amount of tin in the tin-plating layer 12 was set to 11.2 g/m²when tin-plating layer 12 was formed and that the conditions of theelectrolytic treatment for forming the phosphate compound layer and ofthe electrolytic treatment for forming the aluminum-oxygen compoundlayer were as listed in Table 2. The detinning evaluation (modelsolution) was conducted on the surface-treated steel sheet 1 obtained inaccordance with the method aforementioned. The result is listed in Table4.

Comparative Example 3

The surface-treated steel sheet 1 was prepared in the same manner as inExample 1 except that the total energizing time was set to 7.5 secondsand the amount of tin in the tin-plating layer 12 was set to 11.2 g/m²when tin-plating layer 12 was formed and that the conditions of theelectrolytic treatment for forming the aluminum-oxygen compound layerwere as listed in Table 2 without forming the phosphate compound layer20. The detinning evaluation (model solution) was conducted on thesurface-treated steel sheet obtained in the same manner as in Example 8.The result is listed in Table 4.

Reference Example 1

The tin-plated steel sheet 10 was prepared with setting the totalenergizing time set to 7.5 seconds and the amount of tin in thetin-plating layer 12 set to 11.2 g/m² when tin-plating layer 12 isformed. Thereafter, chromium hydroxide was formed by electrolytictreatment on the surface of the tin-plated steel sheet 10 to prepare asurface-treated steel sheet. The detinning evaluation (model solution)was conducted on the surface-treated steel sheet obtained in the samemanner as in Example 8. The result is listed in Table 4. ReferenceExample 1 corresponds to a product of currently commercially availablesurface-treated steel sheets.

TABLE 4 Properties of surface- Surface-treated steel sheet treated steelsheet Amount of Amount of Amount of Amount of

phosphorus aluminum phosphorus aluminum P/ Tin oxide/ (amount of Sn(mg/m

) (mg/m

) (mmol/m

) (mmol/m

) A

tin phosphate remaining (%)) Example 8 0.8 10.6 0.03 0.

0.07 6.9 60 Comparative —

— 0.31 — — 55 Example 3 Reference Chromate 50 Example 1

indicates data missing or illegible when filed

DISCUSSION

As shown in Table 4, the amount of Sn remaining of Example 8, in whichthe tin-plated steel sheet 10 was subjected to cathode electrolytictreatment to form the phosphate compound layer 20 and thealuminum-oxygen compound layer 30 was formed on this phosphate compoundlayer 20, was equivalent to or more than that of Reference Example 1,which is an existing product, and more than that of Comparative Example3, in which the aluminum-oxygen compound layer 30 was formed directly onthe tin-plated steel sheet 10 without forming the phosphate compoundlayer 20. Accordingly, it was confirmed that the surface-treated steelsheet 1 of Example 8, obtained by forming only the phosphate compoundlayer 20 and the aluminum-oxygen compound layer 30 on the tin-platedsteel sheet 10, had a large amount of Sn remaining even in an uncoatedstate without the coating layer 40 provided, thus was excellent incorrosion resistance, and suitable for applications for metal containersand the like used uncoated.

DESCRIPTION OF REFERENCE NUMERALS

-   1 . . . Surface-treated steel sheet-   10 . . . Tin-plated steel sheet-   11 . . . Steel sheet-   12 . . . Tin-plating layer-   20 . . . Phosphate compound layer-   30 . . . Aluminum-oxygen compound layer-   40 . . . Coating layer

1. A method for producing a surface-treated steel sheet comprising:forming a phosphate compound layer on a tin-plated steel sheet by meansof cathode electrolytic treatment, wherein the tin-plated steel sheet isobtained by tin-plating a steel sheet; and forming an aluminum-oxygencompound layer on the phosphate compound layer by means of electrolytictreatment using an electrolytic treatment liquid containing aluminum. 2.The method for producing a surface-treated steel sheet according toclaim 1, wherein the phosphate compound layer is formed on thetin-plated steel sheet by means of the cathode electrolytic treatmentafter anode electrolytic treatment is conducted.
 3. The method forproducing a surface-treated steel sheet according to claim 1, wherein atreatment liquid having a phosphate content of 0.55 g/L or less in termsof phosphorus is used as the electrolytic treatment liquid.
 4. Themethod for producing a surface-treated steel sheet according to claim 2,wherein a treatment liquid having a phosphate content of 0.55 g/L orless in terms of phosphorus is used as the electrolytic treatmentliquid.